SEISMIC WAVES AND EARTH'S INTERIOR STRUCTURE

The basic use since seismic waves are known due to immense depth to the Earth core we are relied upon the indirect study of Earth’s interior. For this purpose we mainly use body waves i.e. S-waves and P-waves as they go deeper into the earth than those of surface waves.

The change in the velocity of the body waves guide us that how the Earth’s interior may look like. We use two processes to proceed towards our knowing:

@  Seismic reflection:

The return of part of the energy of seismic waves to Earth’s surface after the waves bounces off a rock boundary. This process helps geologists know the depth of the boundary.

@  Seismic refraction:

The bending of seismic waves as they pass from one material to another. The seismic refraction happens when the velocity of the waves differ in different layers depending upon their density or strength. Thus this process helps us in knowing of boundaries between different layers.

Now we will study layer to layer property earth’s interior show according to seismic waves.

THE CRUST:

Seismic waves have clearly distinguished between the thicknesses of the crust as thinner beneath the oceans and thicker on the continental area. Moreover the point to ponder upon is that seismic waves travel faster in oceanic crust i.e. 7km/second as compared to in the continental crust i.e. 6km/second. This shows that the rocks present in both the crusts differ from each other. The velocity of waves is greater in the rocks with greater density such as basalt and gabbro and samples taken from the ocean floor also suggest likewise. The velocity of seismic waves on continental crust suggest that rocks present here should be Granitic but as for evidence it is not true as all rocks present on Earth surface are not granitic but also consists of plutonic rocks, gneisses and schist and capped up by sedimentary rocks.

 THE MANTLE:

The data collected through seismic processes shows that mantle is divided into concentric layers. Yet at the same time they show that the layers are in the solid form. The velocity of the P-waves in the upper mantle is 8 Km/second and the increased velocity of seismic waves shows that it is composed of dense igneous rocks known as ultramafic rocks for example peridotite. These rocks are made chiefly of ferromagnesian minerals such as olivine and pyroxene.

Below the uppermost mantle layer a curious boundary is present. Here, the velocity of seismic waves slow down which means that this area have low density and because of this reason it is said that this region is partially melted forming a crystal-liquid slush. This region is known as “low velocity zone” or asthenosphere.

From the data obtained by seismic reflection and seismic refractions clear boundaries at 400 and 670 Km below earth surface has been observed. It is suggested by geologists that if the boundaries between mantle layers represent pressure-caused transformations of minerals, the entire mineral may have the same chemical composition throughout, although not the same mineral composition. For example, at a pressure equivalent to a depth of about 670 Km, the mineral olivine should collapse into the denser structure of the mineral perovskite.

CORE MANTLE BOUNDARY:

At the core mantle boundary the P-wave velocity drops down dramatically and this region is up to 200Km thick and is known as Ultra-low velocity zone (ULVZ). Latest studies have likely tried to explain the reason of this low velocity. According to scientists liquid iron alloys from liquid outer core may react with silicates in the lower mantle to form iron silicates. And this layer is electrically conductive and thus explains decrease in the seismic velocities resulting in ULVZ.

THE CORE:

When we talk about the core of the earth then of course no geologist has ever seen the core but yes the studies done by different methods we have come up to the results about the core of the earth. Seismic data has given us the proof of the presence of the core. While studying seismic waves we observe following phenomena:

When seismic waves are produced during earthquake we notice that they do not reach certain areas on opposite side of the Earth.

@  P-Wave Shadow Zone:

The P-waves generated spread out from a quake until at 103° of arc from the epicenter (area of generation of quake) they suddenly disappear from seismograms. And then at 142° they reappear. The region between 103°-142° is called as P-wave shadow zone.

@  S-Wave Shadow zone:

Now as we know that S-waves can only travel through solids and when it disappears at 103° completely it means that the core of the earth is liquid or at least acts like a fluid. And the area beyond 103° is S-wave shadow zone.

The careful analysis of the seismograms show the refraction of P-waves within the Earth’s core suggesting that core has two parts, a liquid outer core and a solid inner core. Seismic and density data of the core suggests that the composition of core is of largely of iron mixed up with small amount of lighter element such as oxygen, sulfur or silicon.